Method and apparatus for mass spectrometry



Aug. 15, 1944. R. c. LAWLOR v 2,355,658

METHOD AND APPARATUS FOR MASS SPECTROMETRY Filed April 17, 1940 2 Sheets-Sheet 1 MIXER AMPL awe-es 9 a & Fe X o o E O\&::| :1 o g [a o 8' I: o 0 all .1 o o 0 Q O INVENTOR.

REED 6! LA WLOE Aug. 15, 1944. R. c. LAWLOR METHOD AND APPARATUS FOR MASS SPECTROMETRY Filed April 17, 1940 2 Sheets-Sheet 2 kokw w INVENTOR. E550 C. LA WLO/Q A TTORNEY Q Patented Aug. 15, 1944 METHOD AND APPARATUS FOR MASS SPECTROMETRY Reed'O. Lawlor, Pasadena, Calif., assignor to Consolidated Engineering Corporation, Pasadena,

Calif., a corporation or California Application April 17, 1940, Serial No. 330,127

13 Claims The present invention relates to improvements in mass spectrometry. V

In general, a mass spectrometer is more satisfactory than any other instrument for determining the constitution of a gas, for the reason that a mass spectrometer is more sensitive, rapid-and accurate, than any other.

when a gas sample is introduced into a mass spectrometer, it is ionized and by the combined action of electric and magnetic forces the ions formed are segregated according to their massto-charge ratios.

The intensity oi the segregated ion currents produced from a mass spectrum may be used to determine the constitution of the gas mixture under investigation. However, since any particular ion current collected often does not correpond to only a single component in the gas mixture, a complex set of calculations must ordinarily be made on the basis of the various measured ion currents in order to determine the concentrations of the respective individual components in the original gas mixture.

It is clear that the dela and complexity resulting from the need for such calculations is for 25 some purposes undesirable. For example, if it is desired to use a mass spectrometer for continuously determining the composition of a gas mixture flowing through an output line at the termination of a refinery process, it is very impor--'30 tant to be able to continuously indicate and/or record the instantaneous values of the concentrations of the various components yielded by the refinery process.

Accordingly, it is the principal object of my 35 bodying and utilizing my novel method. It is, 45

therefore, to be understood that my method is applicable to other apparatus, and that I do not limit myself in any way to the apparatus of the present application, as I may adopt various other apparatus embodiments, utilizing the method, 60

within the scope of the appended claims.

Referring to the drawings: Fig. 1' shows a general organization of my apparatus, partly in cross-section and partly schematic.

Fig. 2 is. a segmental-view of the ion collecting apparatus taken on the line 2-2 of Fig. 1.

Fig. 3 is a wiring diagram of a preferred means for deriving separate indications of the concentrations of the two components of a binary gas mixture.

Inthe preferred embodiment of my invention a mass spectrometer is connected to a gas line for continuous analysis of the gases flowing. therethrough and the output of the mass spectrometer is applied to a mixing circuit, which serves the purpose of producing a plurality oi currents, each of which is a direct measure of a difierent gas component flowing through said gas line.

Referring to Fig. l: A gas line 2. in which gas is flowing downward as indicated by arrows 4-4, contains therein a gas-collecting tube 6 pointing upstream and a gas discharge tube 8 pointing downstream. A portion of gas flowing through the gas line 2 passes into collecting tube 6 and thence into the sample chamber I0 and returns to the gas line through discharge tube 8.

Valves l2 and I4 may be used to control the speed of flow through chamber iii if desired.

As a result the gas within chamber l0 continuously and accurately reflects the composition of the gas flowing in gas line 2. When valve I6 is open, gas is caused to flow through flrstcapillary l8 in plate 20 into the intermediate chamber 22. A large portion of the gas flowing into chamber 2: may be drawn oil through vacuum line 24 at a rate determined by the setting of control valve 26, thus serving to reduce the gas pressure that would otherwise exist within intermediate chamber 22. A portion of the gas within chamber 22 passes through a second capillary 28 in plate 30 and is thereupon admitted through jet 32 into ionization chamber 34.

The function of the pressure reducing system represented by the two capillaries i8 and 28 and pressure-control valve 26 is simply to limit the amount of gas admitted to ionization chamber 34 to a suitably low value required for the satisfactory operation of the massspectrometer. The pressure within ionization chamber 34 is also determined in part by the rate at which gas is withdrawn through vacuum line 86.

Within ionization chamber 34 is mounted a helical filament type cathode 38, the electron emission from which is determined by a battery 40, and a helical filament anode 4i mounted within cathode 38 and concentric therewith. By means of battery 42, electrode 4| is maintained at a positive potential with respect to cathode 38.

pp Electrons emitted from cathode 38 are drawn towards the center 01' the electrode system 38-, where they bombard and ionize gas molecules contained therein. The positive ions formed are drawn toward collimator slit 50 at the entrance of grounded collimator tube 52 by means of the electric field supplied by battery 54 having its positive end connected to electrode 4| and its negative end connected to metallic chamber 56 and ground 58.

A portion of the positive ions passing through collimator slit also pass through collimator slit 60 in grounded collimator plate 62 thereby forming a heterogeneous ion beam corresponding in content and character to the gas mixture flowing in gas line 2.

Said heterogeneous ion beam, after having been accelerated by the potential between electrode assembly 38- and collimator 52-62, is deflected downward by means of the electric field maintained between plates 64 by battery 66. Said ions pass into the gap I0 between the poles II and I2 (see also Fig. 2) of electromagnet I3.

The strength of the magnetic flux in gap I0 may be set at a predetermined value by adjustment of rheostat I4 which supplies current from battery 15 to coil I6.

Due to the combined action of the electric and magnetic fields, positive ions of various mass-tocharge ratios are brought to a focus at different predetermined positions in the plane of grounded shield I8. Said grounded shield has slits 80, 82, and 84 cut therein at such positions that ions of predetermined mass-to-charge ratios are permitted to pass therethrough and fall upon the corresponding ion collectors 90, 92, and 94, where said ions give up the respective positive charges and produce minute electric currents corresponding in magnitude to the concentration of the respective ions formed within ionization chamber 34.

Referring more particularly to Fig. 2 for details of the ion collecting system, plate I8 is shown secured to the sides of magnetic poles 12 and I4 and metallically connected thereto in order to provide a ground connection for all ions falling on said plate. In front of the slots 80, 82, and 84 are positioned corresponding ion collectors 90, 92, and 94 mounted on plate 18 and insulated intensities of the respective ion currents appearing in the mass spectra of pure samples of said components. One example of how such predetermined fractions are computed is illustrated hereinbelow in connection with the analysis of a mixture of normal butane and isobutane.

In order to achieve the above-described indications of individual components, ion currents produced at collectors 90, 82, and 94 are led into a bank N6 of amplifiers'where they are ampli lied and preferably converted into corresponding A. C. voltages, and said corresponding A. C. voltages appearing across pairs of leads I20, I22, I24 are applied to the mixerj 26 where said potentials are combined in such proportions and phase that separate indications of three components of the original gas mixture are produced in corresponding indicating meters I30, I32, I34. An example of such a mixing system is hereinbelow described in detail in connection with the analysis of a binary mixture containing normal butane and isobutane. While it would be possible to obtain separate indications of the individual components of a mixture by properly combining direct current indications, I prefer to utilize alternating current indications, as alternating currents are easier to manipulate.

In the general organization of apparatus hereinabove described, the application of my invention to the analysis of a tertiary mixture of three hydrocarbon gases having difiering molecular weights has been described. However, it is to be understood that my invention is applicable to the analysis of a gas mixture having a larger number of individual components, and that the number of ion collectors required for the analysis therefrom. Flexible leads I00, I02, and I04 are connected to said collectors at points H0, H2, and H4.

Assume, for the moment, that a three comproduct of butane as well as propane, and that C2Hs+ is an ionization product of all three components, the intensity of the ion currents corresponding to these two ions CzHs+ and'CaHa dov not indicate directly the amounts of ethane and propane present in the mixture. I obtain separate indications of each of the gas components of the mixture by electrically subtracting a predetermined fraction of the C4H1o+ ion current from the C2H6+ and the CaHa+ ion currents and by electrically subtracting a, predetermined fraction of the remaining CaHa+ current from the 'C2Ho current. The values of said predetermined fractions may be determined from the re ative of any mixture should b at least equal to the number of components in said mixture to be separately measured.

In Fig. 3 I have shown a. preferred embodiment of my invention which is applicable to binary mixtures for which two corresponding ion currents are measured and in which the intensity of each ion current is determined in part by each of the components present in said binary mixture.

Such a system as that shown in Fig. 3 is ap plicable, for instance, to the analysis of a mixture of two isomers as normal butane and iso-butane.

'While each of these gases on ionization forms ions of the same mass-to-charge ratio, the relative proportion of the ions formed is characteristic of each. In the table are shown the relative intensities of some of the collected ion currents obtained from these gases under certain conditions of analysis fixed with respect to rate of flow of each component through gas line 2, rate of admission into ionization chamber 34, voltages applied to electrode assembly 38-, etc.

Table MMs Mb Normal Isoge rincipal butane butane Sensitivity current current 2. 90 l. 53 1. 9 3. 58 l. 44 2. 5 0. 046 O. 071 0. 8. 25 5. 78 1. 43 0. 214 0. 498 0. 43 0. 327 0. 191 l. 7 C4H1o 0.666 0.118 5.6

In the table the first column represents the mass-to-charge ratios of the corresponding ions given in the second column, and which are derived from both normaland iso-butanes. The

third and fourth columns indicate the relative .aasaess intensities of'the respective ion currents per unit ions derived from the two gases normal butane and iso butane. I

In the analysis of such a binary mixture, ions of two different predetermined mass-to-charge ratios are caused to fall upon two separate ion collectors 90 and 92. For a binary mixture the third ion collector 84 shown in Fig. 1 is not needed.

The two ions selected for this purpose should have as widely different sensitivity ratios as is consistent with the ultimate sensitivity of the system. Satisfactory operation may be obtained by collecting ions of mass-to-charge ratios of 29 and 40; but where high ultimate sensitivity is not required, ions of mass-to-charge ratios 44 and 58 are most useful because of their wide differences in sensitivity ratios.

Referring to Fig. 3, the ion current discharged on collector 90 flows into the amplifier bank II8 where it passes through a grid resistor I50 and through other elements in the network to ground 58. The potential difierence appearing across resistor I50 is applied to control grid I52 of electrometer tube I54. A corresponding change in anode I58 current causesa corresponding change in potential across the resistor I58 connected between said anode I58 and space-charge grid Suitable operating potentials are applied to the electrodes I52, I58, I80 and filament I82 of electrometer tube I54 by battery I84 through resistors I88, I88, I10, I12, I14, and I18. The values of said resistors are so selected that: First, a suitable negative bias is applied to control grid I52; second, the potential difference appearing across resistor I58'is normally zero when no ion current is being discharged at ion collector 90; and third, the amplification factor of electrometer tube I54 is independent of minor fluctuations in temperature and the electromotive force of battery I84.

The terminals of resistance I58 are connected to the control grids I80 and I82 of pentode vacuum tubes I84 and I88 of a balanced pushpull modulator. The midpoint of resistor I58 is connected to ground 58 through the secondary winding I88 of transformer I90 and also to the cathodes I92 and I94 of corresponding pentodes I84 and I88 through biasing resistors I98 and I98. Voltage is supplied to anodes 200 and 202 and screens 204 and 208 by'battery 208 through resistors 2I0, 2I2, 2I4, and 2I8.

In the preferred form of my invention alternating currents of fixed frequency are supplied from oscillator 2I8 through transformer I90. When no ion current is falling upon ion collector 90 there is no potential difference across resistor I58 and accordingly there is no alternating current potential appearing at the output terminals 220 and 222 of the balanced modulator. However, when positive ions fall on ion collector 90 the potential of grid I80 swings negative and the potential of grid I82 swings positive with respect to ground 58 as a result of the potential appearing across resistor I58. By means of the above described system I am able to produce an alternating current potential appearing across output terminals 220 and 222 in direct proportion to the number of ions per second falling on collector 90.

In similar manner an alternating current potential is developed across the terminals 224 and 228 at the output of a second similar balanced modulator coupled in similar manner to ion collector 92 through electrometer tube 228.

By virtue of the fact that oscillator 2I8 supplies potentials to both balanced modulators through two transformers I80 and 280 connected in series and because of the similarity in characteristics of said two balanced modulators, the alternating currents appearing at the output oi. said two balanced modulators are in phase.

The outputs of said two balanced modulators are applied through coupling condensers 282, 284, 238, and 288 to mixing circuit I28. An alternating current of amplitude I90 corresponding tothe ion current falling on collector 90 flows from terminal 220 through primary winding 282A, center grounded primary windings 284A, and primary winding 28811 to output terminal 222. Similarly an alternating current of amplitude In corresponding to the ion current falling on collector 92 flows from terminal 224 throu h primary 242, center grounded primary winding 244, and primary winding 248 to output terminal 228.

The ratios of the windings of transformers 248 and 248 are so selected that the electromotive forces induced in the secondaries 250 and 252 when added together vectorally or algebraically combine to produce in meter I30 an indication of the concentration of normal butane. Condenser 254 shunted across the series-connected secondaries 250 and 252 is of such a value that the secondary circuits of transformers 248 and 248 are tuned to the frequency of oscillator 2I8.

In similar manner the ratio of the windings 280 and 282 of transformers 288, 288 are adjusted to such values that when the potentials induced in said secondaries are combined in mixer I28, they produce an indication on meter I82 which is a measure of the concentration of isobutane. Condenser 210 serves to tune the series-connected secondaries 280 and 282 to the frequency of oscillator 2 I8.

If ions of masses 29 and 43 are collected by collectors and 92, respectively, then, as will be clear from the table, the two currents I90 and I92 are given by the equations where Xu and X1 are respectively the rates of flow of normal butane and isobutane in the mixture flowing in the line 2., Solving equations (1) and (2) for Xn and X1, it is found that Thus, to obtain separate indications corresponding to the two constituents of the mixture under analysis, transformers 248, 248, 288, and 288 having the relative transformer ratios 0.680, 0.170, 0.970, and 0.421, respectively, are used, and the potentials induced in the two secondary circuits combined in opposite phases (that is, in appropriate multiples of In short, the electrical potentials or currents are combined in phases displaced from each other by multiples of 180. Angles which are multiples of 180 are (for the purposes of this invention) 0, 180, 360 etc.

In the preferred embodiment of my invention illustrated here, I use pentode vacuum tubes as amplifier elements in the balanced modulators because of the high internal resistance of said tubes. The plate resistances 2I4, 2I8, 2I5, and 2 I1 are also of such high values that the mutual coupling between the first pair of secondaries 250 and 252 and the second pair of secondaries 260 and 262 is so low that currents existing in either of the secondary circuits do not produce appreciable electromotive forces in the other of said secondary circuits. This decoupling system is especially useful when current meters are used for indicators I30 and I32.

Usually the value of grid resistor IE is so large that almost all of the potential between grid I52 from the ion collector 90 to ground 58 appears across said grid resistor and accordingly very little ion current potential drop appears across either half of resistor I58. If, however, the potential drop through the two halves of resistor I58 should be of any substantial amount they may be equalized if desired by insertion of an appropriate balancing resistor in either the plate lead 280 or the screen lead 282 as required by the particular values of the other circuit elements. While this is not usually necessary it may be desired for symmetry.

In the above description of my invention it has been assumed that each of the two push-pull modulators described is balanced in such manner that the signals appearing in each output is zero when the corresponding ion currents are zero. However, it is to be understood that this is not absolutely essential to my invention and that many variations of the above-described embodiment of my invention will readily occur to those skilled in the art.

I claim:

1. In a mass spectrometer the combination which comprises a plurality of ion collectors each disposed in said mass spectrometer at such positions as to respectively collect charges carried by ions of different mass-to-charge ratio, at least one electric signal indicator, and electric means connecting each of said collectors to at least one of said indicators, said electric means including signal combining means connecting at least one of said indicators to a plurality of said collectors.

2. In a mass spectrometer the combination which comprises a plurality of ion collectors each disposed in the ion image field of said mass spectrometer in such positions as to respectively collect ions of a different mass-to-charge ratios, a plurality of amplifiers, a common source of alternating current signals connected to the input of said amplifiers, a connection from each of said collectors to one of said amplifiers adapted to vary the amplitude of the alternating current signal appearing at the output of said amplifier in accordance with the amplitude of the respective ion currents collected, alternating current signal combining means connected to the output of said amplifiers, and at least one indicator con-- nected to the output of said combining means.

3. In a method of analyzing a mixture containing a plurality of chemically different components, involving the transformation of a portion of the mixture into a plurality of separate ion beams, each beam comprising ions of a dif ferent mass-to-charge ratio and the intensity of each beam depending on the amount of more than'one of said components present in the mixture, the improvement which comprises converting the respective ion beams into a plurality of electrical currents, electrically combining said electrical currents in such proportions and in such a sign relationship depending on the effect of different components on the intensities of said beams so as to produce a plurality of signals each indicating respectively the amount of a different component of the mixture.

4. In a method of analyzing a mixture containing a plurality of chemicaly different cominto a corresponding plurality of electrical currents, electrically combining the current corresponding to said one ion beam with at least one other of said electrical currents in such proportions and in such a sign relationship depending on the effects of different components on the intensity of said one ion beam so as to produce a signal which indicates the amount of one of the components affecting the intensity of said one ion beam.

5. In a method of analyzing a mixture con taining a plurality of chemically different components, involving the transformation of a portion of the mixture into a plurality of separate ion-beams, each beam comprising ions of a different mass-to-charge ratio and the intensity of each beam depending on the amount of more than one of said components present in the mixture, the improvement which comprises converting the respective ion beams into a plurality of alternating electrical currents of the same frequency, electrically combining said alternating electrical currents in such proportions and in such phase relationships depending on the effect of different components on the intensities of said beams so as to produce a plurality of signals each indicating respectively the amount of a different component of the mixture.

6. A method according to claim 5, in which the electrical currents are combined withrelative phases which are integral multiples of 7. In a method of analyzing a mixture containing a plurality of chemically different components, involving the transformation of a portion of the mixture into a plurality of separate ion beams, each beam comprising ions of a different mass-to-charge ratio, and the intensity of at least one of said ion beams depending on the amount of more than one of said components present in the mixture, the improvement which comprises converting the respective ion beams into a corresponding plurality of alternating electrical currents of the same frequency, electrically combining the current corresponding to said one ion beamwith at least one other of said electrical currents in such proportions and in such phase relationships depending on the effect of different components on the intensity of said one ion beam so as to produce a signal which indicates the amount of one of the components affecting the intensity of said one ion beam.

8. A method according to claim 7 in which the electrical currents are combined with relative phases which are integral multiples of 180.

9. In a mass spectrometer having a source of ions and ion segregating means for separating and focussing ions of different mass-to-charge ratios to produce a plurality of corresponding ion currents, the improvement which comprises a plurality of circuits'operatively associated with the ion segregating means and responsive to different ion currents for producing a corresponding electric current corresponding to the amount of ions of a different mass-to-charge ratio formed at the ion source, at least one electric signal indicator, an electric signal combining means so connecting the plurality of circuits to the indicator and so arranged as to apply to the indicator an electric signal which corresponds to a combination of said electric currents.

10. In a mass spectrometer having a source of ions, ion segregating means for separating and focussing ions of difierent mass-to-charge ratios and a plurality of means for collecting charges carried by corresponding ions of different massto-charge ratios, the improvement which commeans so connecting the plurality of circuits to.

the indicator and soarranged as to apply to the indicator an electric signal which represents a combination of said electric currents.

11. In a mass spectrometer, the combination which comprises an ion current collector responsive to a selected beam of ions produced in the spectrometer, a source of alternating current, and a circuit interconnecting said collector and said source arranged to convert said alternating current into another alternating current modulated in accordance with the intensity of the collected ion current.

12. In a method of analyzing a mixturercontaining a plurality of different components involving the transformation of a portion of the mixture into a plurality of separate ion beams, each of which comprises ions of a different massto-charge ratio, the improvement which comprises introducing a single frequency signal into a plurality of difierent channels, modulating the signals in the respective channels in accordance with the number of ions in the respective ion beams to produce a plurality of modulated currents, and combining the modulated currents in a mixing circuit to produce mixed signals,

13. In a mass spectrometer the combination which comprises an ion current collector, means collected ion current. 

